How Does Electrolyte Type Influence Voltage in Galvanic Cells?

[yucheng]

Homework Statement

Understand the Galvanic Cell, and not let it insult your intelligence

Relevant Equations

N/A

I was watching this video that I found on Youtube.

My question is, does type of electrolyte affect the voltage reading, since it dictates which redox reactions are possible? (here, Na2SO4 instread of CuSO4)

Consider a Mg|Cu electrode pair in a galvanic cell; the reaction is, usually in textbooks, between $\text{Mg}|\text{Cu}^{2+}$, where Cu electrode can, in this reaction, be considered inert. Hence, the voltage measured is actually between $\text{Mg}|\text{Cu}^{2+}$ (with all the salt bride/porous pot), right?

So, does the voltage between Mg|Cu electrode pair with Na2SO4 electrolyte mean? It is not the potential difference calculated from the $E^0$ value of half cell equations of Mg and Cu.

Also,
If you connect 2 different metals with a wire (with no electrolyte), will there still be a natural flow of electrons, albeit only to a small degree?

https://www.quora.com/If-you-connec...ow-of-electrons-albeit-only-to-a-small-degree

But after a flow of electrons, won't the potential difference then become 0? Plus, does this have anything to do with the $E^0$ in the of standard electrode potential (see my argument above about Mg|Cu vs Mg|Cu2+)?

P.S. I am still confused with what actually gives rise to the voltage!

 

[yucheng]

sodium sulphate mentioned at 2:11 in the above video

 

[Borek]

My question is, does type of electrolyte affect the voltage reading, since it dictates which redox reactions are possible? (here, Na2SO4 instread of CuSO4)

It depends. If the electrolyte is inert with regard to the substances taking part in the redox process, it is there only to add conductivity to the solution and doesn't change the potential (to some extent it always does, but let's ignore that for a moment).

It is not the potential difference calculated from the $E^0$ value of half cell equations of Mg and Cu.

E₀ is only for standard conditions, so if you can guarantee all substances are in their standard conditions, the potential difference will be just that: the difference between E₀. For any real solution you should use Nernst equation.

 

[yucheng]

electrolyte is inert

Just curious, here, Mg|Cu dipped in sodium sulphate solution, the electrolyte is not inert, because Mg displaces hydrogen ions in water right?

In this case, the voltage measure, assuming everything is under standard conditions, would be that of the standard electrode potential of Magnesium right?

 

[Borek]

Just curious, here, Mg|Cu dipped in sodium sulphate solution, the electrolyte is not inert, because Mg displaces hydrogen ions in water right?

In this case electrolyte is mostly inert. Mg reacts with the solvent, but that's another thing, it just means the reduction part of the process doesn't involve copper.

And when the whole redox process takes place on the electrode surface it is very difficult (if not impossible) to measure the potential. For measuring you need to separate reduction and oxidation so they take place separately, only then you will be able to plug the voltmeter in the circuit.

 

[yucheng]

For measuring you need to separate reduction and oxidation so they take place separately

Indeed, this is an important point that could have cleared much confusion, if understood early on $\text{:)} \text{)}$

 

[Mike S.]

There are a few different basic points there.

First, E0 is only at standard concentrations and standard temperature and pressure (which might more accurately be called random temperature and pressure since you're lucky if your source knows if they're talking about 1 atm or 1 bar or 25 C or 0 C or some other C, let alone told you). At least the concentrations are at 1 mol/L, except gases which are at 0.04 mol/L because some people don't realize they have a molar concentration. maybe. So ... point is, if you dip solid copper in a solution, a certain number of ions are going to get witched out of it at pretty much any voltage, because there are so many that can leave, and at first none at all that can go the other way. Sheer entropy drives ions (and electrons) to move. (Note you can sometimes visualize this as pressure, because the osmotic pressure from two different ion concentrations or the electrochemical potential energy in a gradient across a selectively permeable membrane is very much the same as the pressure or extractable energy from having two different mol/L concentrations of gas atoms in vacuum with the same geometry. The particles contain an average kinetic (=thermal) energy in each degree of freedom proportional to temperature regardless of what phase they're in)

Also, think of voltage as energy. It is formally J/C, but "coulomb" is just a silly, now formally almost archaic old name for about 10.36 microequivalents = 10.36 micromol of + or - charge. It'd be better to multiply all the voltages by 96485.3 and report them as J/mol of charge, but alas no, we're not there yet. Point is, the electron is sitting in some high nosebleed seat overlooking a metal atom, perhaps indulging itself in pornographic daydreams about putting its nose deep into an oxygen atom starved for companionship, when you provide it with a path to get to some spot closer to a nucleus or closer to a more positive nucleus, and it starts moving on the route that will allow it to release that energy. Note the conductor you don't think about is vastly more complicated than the beginning and end points - the conductor has many states at many different energies available for electrons, and they can be at different levels depending which end of the circuit they're at ...

But a circuit that isn't a circuit doesn't flow long - it's a capacitor that only accepts a little charge. This is because like repels like and the whole force underlying the trip is undone by the force from those who have made it first, like taking a trip to an amusement park only to find the rides are unaccessible. In the cell, this is made up for by letting ions complete the circuit, which can be kind of weird because the circuit is electrons moving one way and ions moving the other way (around a ring-shaped circuit model) or moving together if you look at it as two cells linked together. However you tot it up, the same net positive charge runs all the way around in a loop to keep things moving because the charge doesn't pile up anywhere.

Hope this helps get your intuition started...